Microtubules are a major filament of the cytoskeleton and are involved in various biologic phenomena including mitosis, cell shape determination, cell locomotion and movement of intracellular organelles. Tubulin is one of the major microtubular components. Polymerization and depolymerization of tubulin regulate microtubular dynamics. Microtubules are considered one of the most important molecular targets for cancer chemotherapy.
Antimitotic agents which disrupt microtubules can be classified into two categories based on mechanism of action. These are the vinca alkaloids such as estramustine, rhizoxin and E7010, which inhibit microtubule polymerization, and taxanes such as paclitaxel and docetaxel which promote polymerization of microtubules and enhance microtubule stability.
The anti-mitotic anticancer agent paclitaxel is active against solid tumors. Paclitaxel is a microtubule-disrupting agent that primarily targets tubulin. In the absence of guanosine triphosphate (GTP), paclitaxel induces polymerization and stabilizes tubulin to cold- or calcium-induced microtubule depolymerization, thereby blocking cell cycle in the M phase. Tubulin is a heterodimer that consists of the alpha- and beta-tubulin subunits that form the microtubule.
Clinical trials with taxanes such as paclitaxel and docetaxel have revealed these agents to be effective against several cancers which were advanced or resistant to other anticancer drugs, especially breast cancer, ovarian cancers and non small cell lung carcinoma (NSCLC). With respect to NSCLC, a number of randomized clinical trials have demonstrated that survival in patients with advanced stage III or IV NSCLC can be prolonged with paclitaxel. Preliminary studies with paclitaxel showed response rates of 21% and 24% and an impressive 1-year survival rate of 45% in one trial (Chang et al. J. Natl Cancer Inst. 85:388-394, 1993; and Murphy et al. J. Natl Cancer Inst. 85:384-388, 1993). Paclitaxel is now often used in combination with other cytotoxic drugs including cisplatin and carboplatin in patients with NSCLC.
However, the acquisition of drug-resistant tumor cells is still a major problem in the medical treatment of malignant disease. The hydrophobic nature of drugs such as paclitaxel is known to induce overexpression of the MDR1 gene (Horwitz et al. Monogr J. Natl Cancer Inst. 15:55-61, 1993). However, paclitaxel resistant human lung cancer cells selected in the presence of low levels of paclitaxel do not express MDR1 (Kavallaris et al. J. Clin, Invest. 100:1282-1293, 1997). Further, cells expressing high levels of the multidrug resistance (MDP)-associated protein MRP display no or low resistance to paclitaxel (Zamas et al. Proc. Natl Acad. Sci. USA 91:8822-8826 1994).
It has now been found that mechanisms of drug resistance to anti-cancer agents such as paclitaxel in cancer are related to mutations in the beta-tubulin gene which affect microtubule dynamics.